The overall aim of this research program is to investigate how the time interval between temporally discrete sounds in a sequence influences the fundamental response properties of central auditory neurons. Sounds produced in sequences are common to most bioacoustics signals such as the biosonar sounds used by bats and communication sounds including speech. Recent studies in the inferior colliculus (IC) of bats show a progressive increase in amplitude selectivity of neurons to sounds in a sequence when the rate of acoustic stimulation is increased. Pilot studies have revealed that the rate of stimulation has a more global impact on an IC unit's responses across several dimensions, including the unit's frequency selectivity. In addition, the ability of human listeners to discriminate small differences in sound level improves with increased rate of acoustic stimulation during psychophysical experiments. Therefore, understanding the mechanisms that underlie rate-dependent changes in amplitude and frequency selectivity could not only explain behavioral performance of echo locating bats, but it could also advance our understanding of speech processing in humans. Three specific aims will be addressed. Aim #1 is to characterize quantitatively the response selectivity's of single neurons in the IC to sound frequency when the stimuli are presented at different pulse repetition rates to test the hypothesis that frequency selectivity is dependent upon the rate of stimulation, i.e., the selectivity is greater when the tone pulses are presented at higher repetition rates. Aim #2 is to determine the minimum number of tone pulses in a train required for a change in amplitude selectivity to test the hypothesis that the amplitude and frequency selectivity's of IC neurons to a sound pulse can be enhanced in the presence of as few as one preceding sound pulse. Aim #3 is to study the postsynaptic events that underlie the rate-dependence in amplitude and frequency selectivity's in single IC neurons, using extracellular microiontophoretic injection techniques and intracellular recording to test the hypothesis that inhibition is more robust when sound pulses are presented at high than low repetition rates.